A new campaign by Leo Burnett will try to promote Prozac directly to consumers.

REMEMBER when Johnny Mathis sang ''When Sunny Gets Blue?'' The first campaign for Prozac to be aimed at consumers hopes to reverse that tune.

The campaign, by the Leo Burnett Company in Chicago, uses such weather imagery as dark clouds giving way to bright sunshine -- from a sun that is somewhat evocative of the ''happy face'' symbol that urged everyone to ''Have a nice day'' in the 1970's -- to promote Prozac, the antidepressant treatment that has come to symbolize how consumers in the 1990's try to cure ills with pills.

Until the campaign begins running in August issues of more than 20 magazines, Eli Lilly & Company, which brought out Prozac a decade ago, had advertised the brand solely to health care professionals.

Prozac joins a lengthy list of products available only by prescription that are being advertised directly to consumers as well as to doctors. Often an agency like Burnett, which works for consumer marketers like Kellogg, McDonald's, Oldsmobile and Pillsbury, lands the assignment to create such a campaign.

''Burnett's expertise in being able to connect with the general public will be helpful for us,'' he added.

Initially, Prozac inspired paeans to its palliative properties, like the best seller ''Listening to Prozac'' and a cover article in Newsweek that proclaimed, ''Prozac has attained the familiarity of Kleenex and the social status of spring water.''

But even Kleenex has suffered blows from rival tissues and spring water runs only so deep. While Prozac remains the most popular antidepressant in the United States, with sales of more than $1.7 billion last year, doctors have been increasingly writing perscriptions for competitive brands like Zoloft, sold by Pfizer Inc., and Paxil, from SmithKline Beecham P.L.C.

The goal of the Prozac print campaign, with a budget estimated at $15 million to $20 million, is not to urge users of those medications to switch Prozac by asking them to put on a happy face, look for the silver lining or keep their sunny sides up. Rather, the ads, which carry the cheery theme ''Welcome back,'' are meant to stimulate demand for Prozac by urging people who are not being helped for depression -- or receiving less strong medicine than they may need -- to seek treatment.

''There has been lots of publicity about Prozac, but it hasn't made the connection with the kind of people who need help,'' Mr. Clark said, ''because there are still data, fairly compelling, showing that two out of three people in a depressed condition are not getting treatment.''

''There are two messages here,'' he added: ''Depression hurts. And there are options available, of which Prozac is one.''

Mr. Clark spoke in an interview in Manhattan at which he was accompanied by Mary O. Bishop, executive vice president at Leo Burnett U.S.A.

''We talked to 900 consumers in great depth,'' she added, ''spending intensive time with depressed patients seeking treatment.''

One finding, according to Ms. Bishop, was that the patients said they ''come from a boomer generation where parents said, 'Why don't you pick yourself up?' ''

''So as a medical area,'' she added, ''this is still in the closet.''

The ads play down notions that Prozac is ''a 'happy pill' ''; they even caution, ''Remember, Prozac is prescription medicine, and it isn't right for everyone.''

The ads do, however, couch some qualifications about Prozac in, well, sunny terms. For instance, at one point the ads assert: ''It's not a tranquilizer. It won't take away your personality. Depression can do that, but Prozac can't.'' (The ads also list ''mild side effects'' of Prozac like upset stomachs and anxiety. There is also dense text, required by law, with more detailed data.)

The ads will appear, Ms. Bishop said, in magazines like Cosmopolitan, Ebony, Good Housekeeping, The Ladies' Home Journal, Marie Claire, Men's Health and Sports Illustrated.

The fate of the campaign, which Ms. Bishop said was scheduled to run for three to six months, is important to Burnett because its performance is being scrutinized after a recent spate of news gloomy enough to perhaps send even Pollyanna to ask her doctor about Prozac.

Those developments included the toppling of two top executives, the loss of big creative assignments, the failure to advance in important reviews and the departure of the account of United Airlines.

And the trade publication Adweek reported this week that the McDonald's Corporation would stage a creative ''shootout'' between Burnett, which has part of the McDonald's account, and the fast-food company's other national agency, the DDB Needham Chicago unit of Omnicom Group. The competition is intended to come up with ways to reverse McDonald's flagging fortunes in the domestic fast-food wars.

Burnett joined the Lilly roster a month ago when it was awarded the Prozac assignment after a review. Ads aimed at doctors, produced by medical agencies, continue.